Cylindrical oxide fuel pellets are pressed form granulated uranium dioxide powder. The pellets are sintered in a hydrogen atmosphere furnace to obtain the required density and are finely ground to obtain the correct diameter. The grinding operation is required in order to carefully control the clearance between the pellet and the tubular cladding, since this clearance determines the heat transfer characteristics of the fuel pin. During the grinding operation, however, a greasy residue of the order of 2 .mu.m may become fixed to the surface of the fuel pellet.
A critical operation in the assembly of an oxide fuel pin is the process of inserting the finished fuel pellets into the fuel tube. The insertion problem is concerned with pellet jamming as a result of the formation of chips, debris and the like.
The total length of fuel pellets within a fuel pin may exceed 3 meters and, in some designs, 4 metres. It may be required to fill a fuel tube with the correct number of pellets within a short space of time, perhaps of the order of 30 seconds The fuel pellets may typically be manufactured in long and short lengths, nominally 13 mm and 10 mm. The use of both long and short pellets enables the correct pellet stack length to be made up for a particular fuel pin. Pellet jamming is likely to have a major impact upon the production of fuel pins since human access into the assembly areas is extremely difficult and the plant/equipment is normally operated remotely.
Uranium dioxide pellets are susceptible to capping, a term used to describe damage to the integrity of the pressing, and chips are easily detached. As the pellets are loaded into the tube, the tube is vibrated simultaneously in both the horizontal and vertical planes, in order to complete the transfer of the pellets once they are inside the tube. It frequently happens that the griding residue which has adhered to the pellet becomes transferred to an orifice through which the pellets pass prior to entry into the fuel tube. Over time this residue builds up and reduces the diameter of the orifice, eventually causing an obstruction which requires manual intervention to clear. A further problem associated with physical contact of the pellet and the loading equipment is encountered when chip debris becomes detached from a pellet during its passage through the orifice. The chip creates a wedge preventing the remaining pellets from being loaded.
In order to reduce or eliminate the above-mentioned problems, possible proposals which have been considered include the following:
1. Gravity loading the pellet into a vertically orientated fuel tube. Normally, height restrictions will preclude this possibility.
2. Positive air pressure to blow pellets into the tube. The use of compressed air will blow uranic contamination around the plant and, since the non-entry end of the tube is sealed, this approach is impractical.
3. Using two parallel wires to transport the pellets towards the tube. This method is in current use. The tension in the wire must be constantly checked to ensure the pellets are correctly supported. Otherwise they fall between the wires and must then be manually retrieved When the pellets arrive at the fuel tube they must be physically pushed into the open end.
4. Use of a robotic manipulator to "pick and place" the pellets. The large quantity of pellets which need to be transferred and the time taken for completion of such an operation make this approach impractical
5. Cushion Transfer. This method of transporting pellets using vibration and Floatex type carpet strips is in current use. However this process can release small numbers of fibres which must be prevented from becoming entrained within the fuel pellet stack. This method of trer is therefore not considered appropriate in the immediate vicinity of stack loading.
6. Removal of the end cap from the sealed end of the fuel tube and loading pellets into both ends. The resultant overall manufacturing changes would be relatively so significant hat this approach can not be contemplated In addition, it is necessary, at least for certain overall processes, to use fuel tubes sealed at one end. Removal of the end cap would involve unacceptable redesign.